Newt Gingrich continued his full-throttle emphasis on immigration on Thursday in Iowa, countering opponents who have accused him of embracing amnesty for illegal immigrants.

Gingrich signed a pledge to build a fence along the entire 2,000-mile stretch between Mexico and the United States by the end of 2013.

Building the fence could cost taxpayers billions of dollars, not including annual maintenance expenses. But Gingrich told The Des Moines Register in an interview that those costs could be trimmed as much as 95 percent by simply eliminating all federal regulations for the fence’s construction.

He did not explain how he arrived at that estimate and his staff was unable to pinpoint the information Thursday.

“Remember, we built the Pentagon for almost nothing because we didn’t go through all the modern baloney,” Gingrich said.

Such federal regulations are intended to protect water quality, prevent ground pollution and ensure worker safety — all items generally seen as critical to human health.

Several immigration reform advocates said Thursday that while they agree with Gingrich that action is needed, they doubt his cost-saving ideas and whether such a fence would be effective.

A better idea would be to invest the billions of dollars in increased security and screening at the nation’s ports of entry, where the majority of illegal immigration and drug smuggling occurs, said William Moore, a spokesman for the Texas Border Coalition. The nonpartisan group of mayors and local officials represents more than 6 million people living along the border.

Moore also contends that building the fence would be difficult if not impossible because of the region’s harsh landscape. Because of flood plains, some U.S. farmers and their homes would likely be on the Mexican side of the fence, creating numerous safety and property rights issues, he noted.

“Erecting a fence on top of 400 feet of sheer vertical rock at a cost of several million dollars a mile is folly. It’s waste,” Moore said.

Gingrich is the second presidential candidate to sign the pledge drafted by Americans Securing the Border. U.S. Rep. Michele Bachmann, a Republican from Minnesota, has also signed.

Gingrich signed the pledge after an event at Nationwide Insurance in Des Moines, and compared the issue to World War II.

“In three years and eight months we defeated fascist Italy, Nazi Germany and imperial Japan because we were a serious country,” Gingrich said. “In 25 years we haven’t been able to build a fence and border because we’re not a serious country.”

Larry Ginter of Iowa Citizens for Community Improvement called the fence “an apartheid wall.”

“We need comprehensive immigration reform with a clear pathway to citizenship for the everyday people and hardworking families that keep our economy and our small towns alive,” said Ginter, a farmer from Rhodes.

More than 600 miles of fence or barriers have already been built along the 2,000-mile stretch under George W. Bush’s presidency and continued during President Obama’s term. Dozens of federal regulations were waived for that portion, which cost roughly $2.4 million a mile to construct.

The Congressional Budget Office has estimated construction costs at $3 million a mile, plus annual maintenance of roughly 15 percent of the overall project costs.

“It’s a backwards way of looking at things,” Mary Giovagnoli, the director of the Immigration Policy Center in Washington, D.C., said about the idea to build a fence. “The real issue is whether we have an immigration system out of touch with Americas needs.”

Some of his rivals criticized Gingrich, a former U.S. House speaker, after he said during a debate last week that he would push for a humane approach toward immigration. He argued there should be a system to allow some longtime immigrants to attain a legal status short of citizenship.

Bachmann said that by opposing mass deportation, Gingrich is embracing amnesty, an accusation Gingrich denies.

Gingrich is now the Republican presidential frontrunner in some national polls, including one released by Rasmussen Reports on Thursday showing him at 38 percent, followed by former Massachusetts governor Mitt Romney with 17 percent.

“I’m going to be the nominee,” Gingrich told ABC News on Thursday. “It’s very hard not to look at the recent polls and think that the odds are very high I’m going to be the nominee.”